Qualities of synthetic quartz glass substrates include the size and density of defects on substrates, flatness, surface roughness, photochemical stability of material, and surface chemical stability. Of these, the quality relating to defects on substrates encounters increasingly rigorous requirements in accordance with the trend of IC technology toward finer feature size processing and of display panels toward larger size.
While improvements in the defective quality of synthetic quartz glass substrates have been continuously made, the substrates used so far in the semiconductor technology, for example, are those substrates where recessed defects with a size of less than about 0.3 μm are substantially present. This is especially because a detection probability of defects with a size of up to 0.5 μm is low upon visual inspection under a collective lamp at an illuminance level causing no hygienic issue, or defect inspection by an automatic defect inspector as described in JP-A S63-200043 (Patent Document 1) and JP-A S63-208746 (Patent Document 2). This leads to a delay in the substrate quality improving approach.
Under the background, a high-sensitivity defect inspector was recently developed. Research works have progressed for controlling microscopic surface defects using this inspector. While JP-A S64-40267 (Patent Document 3) describes a process of polishing a glass substrate with colloidal silica to produce a precise mirror finish surface, this was found unsatisfactory as the microscopic defect controlling process because an analysis of surface defects by said high-sensitivity defect inspector demonstrated the presence of microscopic raised/recessed defects. In JP-A 2002-030274 (Patent Document 4), colloidal silica is described as the polishing slurry for aluminum disks and glass hard disks. It is described that the preferred SiO2 concentration is in a range of 1 to 30% by mass, and Examples carry out polishing with a polishing slurry having a SiO2 concentration of 10% or 14% by mass.
However, when synthetic quartz glass substrates are polished in the above-described concentration range, numerous defects with a size of up to 0.5 μm are generated. Likewise in JP 2987171 (Patent Document 5) and JP-A 2001-003036 (Patent Document 6) relating to a colloidal silica-based polishing slurry as the polishing slurry for silicon wafers, substantially the colloidal silica-based polishing slurry is diluted to a SiO2 concentration of up to 10% by mass prior to use. It is then undesirable as the polishing slurry for photomask-forming glass substrates.
JP-A 2004-98278 describes that microscopic raised defects can be eliminated using high purity colloidal silica near neutrality. In fact, it is impossible to use such colloidal silica in the neutral range in a consistent manner because even those high purity products having minimal contents of metals and other impurities tend to gel or thicken or change the particle size distribution of abrasive grains with iteration of polishing.
Accordingly, such a process has a serious issue that recycling of the polishing slurry for repeated use is difficult, and inevitably the polishing slurry has to be utilized as a one-way stream, which is unfavorable in economical and environmental aspects.
Also, in the case of liquid crystal substrates, for example, further defect control is desired because photomask-forming synthetic quartz glass substrates of larger size are used as the size of liquid crystal panels increases.